1. Field of the Invention
The invention relates generally to management of logical volumes in a storage system, and more specifically relates to techniques for quickly transferring ownership of a logical volume from one storage controller to another storage controller.
2. Related Patents
This patent application is related to the following commonly owned United States patent applications, all filed on the same date herewith and all of which are herein incorporated by reference:                U.S. patent application Ser. No. 13/432,131 (filed Mar. 28, 2012), entitled METHODS AND STRUCTURE FOR TASK MANAGEMENT IN STORAGE CONTROLLERS OF A CLUSTERED STORAGE SYSTEM;        U.S. patent application Ser. No. 13/432,213 (filed Mar. 28, 2012), entitled METHODS AND STRUCTURE FOR DIRECT PASS THROUGH OF SHIPPED REQUESTS IN FAST PATH CIRCUITS OF A STORAGE CONTROLLER IN A CLUSTERED STORAGE SYSTEM;        U.S. patent application Ser. No. 13/432,223 (filed Mar. 28, 2012), entitled METHODS AND STRUCTURE FOR LOAD BALANCING OF BACKGROUND TASKS BETWEEN STORAGE CONTROLLERS IN A CLUSTERED STORAGE ENVIRONMENT;        U.S. patent application Ser. No. 13/432,232 (filed Mar. 28, 2012), entitled METHODS AND STRUCTURE FOR IMPLEMENTING LOGICAL DEVICE CONSISTENCY IN A CLUSTERED STORAGE SYSTEM;        U.S. patent application Ser. No. 13/432,238 (filed Mar. 28, 2012), entitled METHODS AND STRUCTURE FOR IMPROVED I/O SHIPPING IN A CLUSTERED STORAGE SYSTEM;        U.S. patent application Ser. No. 13/432,220 (filed Mar. 28, 2012), entitled METHODS AND STRUCTURE FOR MANAGING VISIBILITY OF DEVICES IN A CLUSTERED STORAGE SYSTEM;        U.S. patent application Ser. No. 13/432,150 (filed Mar. 28, 2012), entitled METHODS AND STRUCTURE FOR IMPROVED BUFFER ALLOCATION IN A STORAGE CONTROLLER; and        U.S. patent application Ser. No. 13/432,138 (filed Mar. 28, 2012), entitled METHODS AND STRUCTURE FOR RESUMING BACKGROUND TASKS IN A CLUSTERED STORAGE ENVIRONMENT.        
3. Discussion of Related Art
In the field of data storage, customers demand highly resilient data storage systems that also exhibit fast recovery times for stored data. One type of storage system used to provide both of these characteristics is known as a clustered storage system.
A clustered storage system typically comprises a number of storage controllers, wherein each storage controller processes host Input/Output (I/O) requests directed to one or more logical volumes. The logical volumes reside on portions of one or more storage devices (e.g., hard disks) coupled with the storage controllers. Often, the logical volumes are configured as Redundant Array of Independent Disks (RAID) volumes in order to ensure an enhanced level of data integrity and/or performance.
A notable feature of clustered storage environments is that the storage controllers are capable of coordinating processing of host requests (e.g., by shipping I/O processing between each other) in order to enhance the performance of the storage environment. This includes intentionally transferring ownership of a logical volume from one storage controller to another. For example, a first storage controller may detect that it is currently undergoing a heavy processing load, and may assign ownership of a given logical volume to a second storage controller that has a smaller processing burden in order to increase overall speed of the clustered storage system. Other storage controllers may then update information identifying which storage controller presently owns each logical volume. Thus, when an I/O request is received at a storage controller that does not own the logical volume identified in the request, the storage controller may “ship” the request to the storage controller that presently owns the identified logical volume.
FIG. 1 is a block diagram illustrating an example of a prior art clustered storage system 150. Clustered storage system 150 is indicated by the dashed box, and includes storage controllers 120, switched fabric 130, and logical volumes 140. Note that a “clustered storage system” (as used herein) does not necessarily include host systems and associated functionality (e.g., hosts, application-layer services, operating systems, clustered computing nodes, etc.). However, storage controllers 120 and hosts 110 may be tightly integrated physically. For example, storage controllers 120 may comprise Host Bus Adapters (HBA's) coupled with a corresponding host 110 through a peripheral bus structure of host 110. According to FIG. 1, hosts 110 provide I/O requests to storage controllers 120 of clustered storage system 150. Storage controllers 120 are coupled via switched fabric 130 (e.g., a Serial Attached SCSI (SAS) fabric or any other suitable communication medium and protocol) for communication with each other and with a number of storage devices 142 on which logical volumes 140 are stored.
FIG. 2 is a block diagram illustrating another example of a prior art clustered storage system 250. In this example, clustered storage system 250 processes I/O requests from hosts 210 received via switched fabric 230. Storage controllers 220 are coupled for communication with storage devices 242 via switched fabric 235, which may be integral with or distinct from switched fabric 230. Storage devices 242 implement logical volumes 240. Many other configurations of hosts, storage controllers, switched fabric, and logical volumes are possible for clustered storage systems as a matter of design choice. Further, in many high reliability storage systems, all the depicted couplings may be duplicated for redundancy. Additionally, the interconnect fabrics may also be duplicated for redundancy.
While clustered storage systems provide a number of performance benefits over more traditional storage systems described above, the speed of a storage system still typically remains a bottleneck to the overall speed of a processing system utilizing the storage system.
For example, in a storage system, data describing logical volumes provisioned on a plurality of storage devices may be stored in Disk Data Format (DDF) on the storage devices. DDF data (or other similar metadata) for a volume describes, for example, physical and virtual disk records for the volumes. Whenever a storage controller assumes ownership of a logical volume, DDF data is processed into a metadata format native to the storage controller. This is beneficial because relevant data is more easily accessible to the storage controller in the native format. Additionally, the native format data is typically a substantially smaller size than the DDF data because it describes fewer logical volumes. Unfortunately, processing the DDF data is an intensive process that delays the processing of incoming host I/O requests directed to the volume. Reading the DDF metadata from the storage devices can consume significant time. Further, the processing to extract data from the read DDF metadata and form desired native format metadata also consumes significant time. This in turn may undesirably reduce the speed of the clustered storage system.
Thus it is an ongoing challenge to enhance the speed at which ownership of a logical volume can be transferred.